Conventional imaging circuits typically use active pixel sensor cells to convert light energy into electrical signals. Each of the active pixel sensor cells generally includes a photoreceptor with several associated transistors that provide several pixel functions including signal formation, reset, and amplification. In a color imager, separate pixels are used for receiving each band of light, such as those corresponding to the primary colors, red, green, and blue. The responsivity of a pixel varies with the specific color of light that is being captured. For example, in a system employing red, green, and blue color pixels, having a uniform integration time for each pixel and a typical scene being imaged; the output signal of a pixel for an amount of light received will vary as a function of the responsivity of the pixel to the imaged color. Correspondingly, the signal to noise ratio (S/N) of the pixels will vary as a function of the responsivity to the imaged color. Typically, blue pixels are less responsive than red and green pixels, causing the S/N of the blue pixels to be less than the S/N of red and green pixels. In addition to differences in S/N, there are differences in saturation of the pixels. Specifically, when capturing an image with equal amounts of red, green, and blue light, the storage capacitance associated with the pixels having the greater sensitivity (the red and green pixels) will reach a maximum capacity of stored photoelectrons first, saturating the pixel.
Separate gain elements for corresponding spectral band channels can be used to equalize the output signals of the different color sensors to compensate for differences in responsivity. However, the gain elements increase the cost of the imager, require increased space, and have no effect on the differences in S/N for the different color pixels.
A macro pixel is provided. The macro pixel includes at least two color pixel elements. Each color pixel element includes a photoreceptor that in response to receiving light, generates an output signal that is indicative of the quantity of light photons received. A first of the color pixel elements is configured to receive a first color. The photoreceptor of the first of the color pixel elements has a first geometry and a responsivity to light that is a function of the first geometry of the photoreceptor such that the responsivity of the output signal of the photoreceptor to the first color is controllable by changing the first geometry. A second of the color pixel elements is configured to receive a second color. The photoreceptor of the second of the color pixel elements has a second geometry and a responsivity to light that is a function of the second geometry such that the responsivity of the output signal of the photoreceptor to the second color is controllable by changing the second geometry.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.